Method for the production of amines

ABSTRACT

The invention provides a process for preparing amines by catalytic hydrogenation of nitroaromatics and subsequent removal of the catalysts from the reaction mixture, which contains at least one aromatic amine and water, which comprises carrying out the removal of the catalysts by means of membrane filtration, which is carried out at a pressure on the suspension side of from 5 to 50 bar, a pressure difference between the suspension side and the permeate side of at least 0.3 bar and a flux rate on the suspension side of from 1 to 6 m/s.

The present invention relates to a continuous process for preparingamines, in particular aromatic amines, by catalytic hydrogenation of thenitro compounds on which the amines are based.

The preparation of amines, in particular of aromatic mono- and/orpolyamines, by catalytic hydrogenation of the corresponding mono- and/orpolynitro compounds has been known for some time and described manytimes in the literature.

The customary industrial preparation of the aromatic mono- and/orpolyamines by reaction of nitro compounds with hydrogen liberates aconsiderable amount of heat. In industry, the hydrogenation is thereforeusually carried out at very low temperatures in the liquid phase in thepresence of hydrogenation catalysts. The compound to be reduced is mixedwith the catalyst in a solvent and reduced batchwise in an autoclave orcontinuously in a loop reactor, a bubble column or a reactor battery.These known processes have a number of disadvantages, for example thenecessity to remove the deactivated catalyst fractions, in particular incontinuous processes, which results in catalyst losses. Furthermore, theside reactions which frequently occur result in the formation ofinterfering substances, for example tar-like constituents, which leadsto reductions in yields, and are a problem in many processes usedhitherto.

EP-A-634 391 describes a process for hydrogenating aromatic polynitrocompounds to give amines, which is said to minimize the abovementionedproblems of hydrogenation of aromatic polynitro compounds throughtechnological optimization using a loop-Venturi reactor equipped with anejector, coupled with specific conditions, such as a precise circulationvolume ratio, precise energy input and a precisely adjusted hydrogenvolume flow rate. The catalysts used are known hydrogenation catalysts,preferably metals from transition group VIII of the Periodic Table, inparticular Raney iron, Raney cobalt and Raney nickel.

This process may, as a result of a heat exchanger for dissipating theheat of reaction being located outside the loop reactor, give rise tooverheating in the ejector and in the reactor, which results inimmediate onset of side reactions such as ring hydrogenations,hydrogenolytic dissociations or the formation of high molecular weight,tar-like products which coat the catalyst surface. In addition, a purebubble-column character relating to the flow and residence timebehavior, which involves random small- and large-volume vortexes withcomparatively low mass transfer performance, becomes established in thereactor volume outside the ejector. There is therefore virtually nosignificant improvement in hydrogenation yields, hydrogenationselectivity and space-time yield in this process. In addition,circulation by pumping of the entire reaction mixture subjects thecatalyst to high mechanical stresses, which again results in a reducedon-stream time of the catalyst.

WO 00/35852 describes a process for preparing amines by hydrogenation ofnitro compounds. This process involves carrying out the reaction in avertical reactor equipped with a downwardly pointing jet nozzle, throughwhich the reactants and also the reaction mixture are fed, an externalcircuit, through which the reaction mixture is fed into the jet nozzle,and also a flow reversal means at the lower end of the reactor. Thedischarge of the end product is preferably effected via a removal unitfor the catalyst. Suggested examples of useful removal units includesettlers, filter units and centrifuges.

This process allows the selectivity and the space-time yields ofhydrogenations to distinctly increase. However, for industrial scalehydrogenation, a further improvement of the process is desirable. A veryhigh degree of removal of the hydrogenation catalyst used is inparticular decisive for the economic viability of the process. Completeremoval of the catalyst from the reaction mixture discharged from thereactor simplifies the workup of the end product. The removed catalystmay be fed back into the reactor and therefore does not need to bereplaced by fresh catalyst. This is particularly important when noblemetal catalysts are used.

It is known that catalysts may be removed by means of a crossflowfiltration. This method of removal leads to a particularly gentleremoval of the catalyst.

DE 32 45 318 discloses carrying out the removal of catalysts ingas/liquid reactions by means of a microfilter operated according to thecrossflow principle. In order to minimize the stress on the catalyst,the filtration is operated at pressures of at least 10 bar on thesuspension side and pressure differentials between the suspension sideand the filtrate side of at maximum 6 bar and also temperatures in therange from 80 to 200° C.

DE 30 40 631 describes the removal of catalysts from reaction mixturesby means of membrane filtration and mentions that this process may alsobe used in the hydrogenation of nitroaromatics. The filters used arehollow fibers. The filtration is carried out at very low temperatures.

It is an object of the invention to develop a process for removingcatalysts in the hydrogenation of nitroaromatics to give aromatic amineswhich allows a complete and gentle removal of the catalysts and allowsthe removed catalyst to be returned from the separating step into thereactor in its entirety.

We have found that, surprisingly, this object is achieved by carryingout the removal of the catalyst by means of a crossflow filter, which isembodied by a membrane filter, at a pressure on the suspension side offrom 5 to 50 bar, preferably from 10 to 30 bar, a pressure differencebetween the suspension side and the permeate side of at least 0.3 barand a flux rate on the suspension side of from 1 to 6 m/s.

The invention accordingly provides a process for preparing amines bycatalytic hydrogenation of nitroaromatics and subsequent removal of thecatalysts from the reaction mixture, which contains at least onearomatic amine and water, which comprises carrying out the removal ofthe catalysts by means of membrane filtration, at a pressure on thesuspension side of from 5 to 50 bar, preferably from 10 to 30 bar, apressure difference between the suspension side and the permeate side ofat least 0.3 bar and a flux rate on the suspension side of from 1 to 6m/s.

For the purposes of the present invention, the suspension side refers tothe side of the membrane filter on which the catalyst-containing mixtureis located, and the permeate side to the side of the membrane filter onwhich the catalyst-free mixture is located.

To carry out the process according to the invention and recover acatalyst-free product stream, the effluent from the hydrogenationreactor is brought into contact with a membrane and permeate (filtrate)is removed on the reverse side of the membrane at a lower pressure thanon the side on which the catalyst-containing reaction mixture islocated. This provides a catalyst concentrate (retentate) which may bereturned to the synthesis reactor without further workup and a virtuallycatalyst-free permeate which comprises the reaction product, thearomatic amine in the process according to the invention, and also waterand, if used, solvent.

The filtration according to the invention may be carried outcontinuously or batchwise.

The continuous operation of the process involves at least a partialstream of the reaction mixture being constantly passed through amembrane filter. When this embodiment of the process according to theinvention is used, preference is given to locating the membrane filterin the external circuit of a circulation reactor. Preference is given tothis embodiment of the process according to the invention.

The batchwise operation of the filtration according to the inventioninvolves conducting the reaction mixture through a hookup purificationstage comprising a membrane filter and its own circulation pump. Anotherembodiment of the batchwise filtration involves the reaction mixturebeing passed through a membrane filter after the reaction. Thisembodiment is less preferred, since the catalyst separated off in thiscase has to be concentrated more highly.

The filter membranes used in the process according to the invention,depending upon the particle size of the catalyst used, preferably havepore diameters in the range from 10 nm to 20 μm, in particular in therange from 50 nm to 10 μm and preferably from 100 nm to 5 μm.

The separating layers of the filter membranes may comprise organicpolymers, ceramic, metal, carbon or combinations thereof and must bestable in the reaction medium and at the process temperature. Formechanical reasons, the separating layers are generally applied to amono- or multilayer porous substructure, which may be of the samematerial as the separating layer or of at least one differing material.Preference is given to inorganic membranes on account of high synthesistemperature and the associated high temperature of the filtered reactionmixture. Examples include metal separating layers and metalsubstructures, ceramic separating layers and metal, ceramic or carbonsubstructures, polymer separating layers and polymer, metal, ceramic orceramic on metal substructures. Examples of ceramics which are usedinclude α-Al₂O₃, γ-Al₂O₃, ZrO₂, TiO₂, SiC or mixed ceramic materials.Examples of polymers which may be used include polytetrafluoroethylene,polyvinylidene fluoride (PVDF), polysulfone, polyethersulfone,polyetheretherketone and polyamide.

The membranes are customarily mounted in pressure-rated casings whichallow separation of the retentate (catalyst-containing) from thepermeate (catalyst-free filtrate) under the pressure conditionsnecessary for the filtration. The membranes may have flat, tubular,multichannel element, capillary or wound geometries, for whichappropriate pressure casings which allow separation of the retentatefrom the permeate are available. Depending on the area requirements, afilter element may comprise plural channels. More than one of theseelements may also be combined within a casing to give a module.

In a preferred embodiment, metal membranes are used which are welded tothe casings.

Preference is given to operating the process so that as far as possibleno cake layers form on the suspension side of the membrane. Ifdisrupting cake layers form, through which the filtration iscompromised, it is possible to remove these by flow reversal betweensuspension side and permeate side. The flow reversal can be achieved inparticular by increasing the permeate pressure to above the retentatepressure.

Optimal transmembrane pressures between the retentate and permeate,depending on the diameter of the membrane pores, the hydrodynamicconditions which influence the buildup of cake layers, the mechanicalstability of the membrane and the operating temperature, and the type ofmembrane, are substantially at least 0.3 bar, in particular from 0.5 to50 bar, preferably from 1 to 25 bar.

Relatively high transmembrane pressures usually lead to relatively highpermeate fluxes. Since the synthesis effluent is usually introduceddirectly to the membrane filtration step at the synthesis presssure, thetransmembrane pressure may be reduced, by increasing the permeatepressure, to a value smaller than the synthesis pressure.

Since the synthesis temperature is predetermined by the process and isabove 80° C., the membrane has to be stable at this temperature.Relatively high temperatures lead in principle to relatively highpermeate fluxes and are therefore preferred.

When, in specialized applications of the process according to theinvention, membranes have to be used which are unstable at thesetemperatures, the reaction mixture has to be cooled before thefiltration and the retentate before introduction into the reactor has tobe heated again. This embodiment is not preferred.

The achievable permeate fluxes depend strongly upon the membrane typeand geometry used, the process conditions, the suspension composition,the catalyst concentration and catalyst type. The fluxes are customarilyfrom 20 to 500 kg/m²/h.

The process according to the invention allows a catalyst retentionof >99% to be achieved.

The process according to the invention allows all catalysts which may beused for the hydrogenation of nitroaromatics to be removed. Usefulcatalysts include metals of transition group VIII of the Periodic Table,which may be applied to support materials such as carbon or oxides ofaluminum, silicon or other materials. Preference is given to using Raneynickel and/or supported catalysts based on nickel, palladium, iridiumand/or platinum on carbon supports. The process can particularlyadvantageously be used for removing catalysts in which little of thehighly fused by-products, frequently referred to as “tar”, occur. Thistar may cause blockages of the membrane used and reduce the lifetime ofthe filter. Since noble metal catalysts operate particularly selectivelyand the hydrogenation of nitroaromatics only forms very little highmolecular weight tar, the removal of the noble metal catalysts by meansof membrane filtration is particularly advantageous.

The average particle size of the catalysts used is usually in the rangefrom 10 nm to 200 μm, in particular in the range from 50 nm to 100 μmand preferably from 100 nm to 30 μm.

The hydrogenation of the aromatic nitro compounds may be effected by thecustomary and known processes.

This involves, independent of the type of the nitro compounds used, apressure of from 5 to 50 bar, preferably from 10 to 30 bar, and anoperating temperature of from 80 to 200° C., preferably from 100 to 180°C., being maintained with preference in the reactor.

The mono- and/or polynitro compound may be used in pure form, as amixture with the corresponding mono- and/or polyamine, as a mixture withthe corresponding mono- and/or polyamine and water or as a mixture withthe corresonding mono- and/or polyamine, water and a solvent, inparticular an alcohol. The aromatic mono- and/or polynitro compound isintroduced to the mixture in highly divided form. The reaction mixturewhich leaves the reactor comprises water which is by-produced during thehydrogenation.

The reactors used include the customary and known hydrogenationreactors. Examples include stirred tanks, bubble columns, which maycontain packings, or loop reactors, such as loop-Venturi reactors or jetloop reactors having internal and external circuits, as described, forexample, in WO 00/35852.

The process according to the invention may particularly advantageouslybe applied to the removal of catalysts when loop reactors having anexternal circuit are used. In this case, the membrane filter is locatedin the external circuit.

A particularly preferred embodiment of the process according to theinvention involves the use of a hydrogenation reactor as described in WO00/35852. This embodiment does not require an additional pump for themembrane filtration, since the necessary pressure on the suspension sidemay be maintained by the pump for the external circuit. This allows theprocess to be distinctly simplified. Also, the use of such reactorsmakes complete conversion of the nitroaromatics possible so that thesubsequent workup after the complete removal of the catalyst becomesparticularly simple. This process utilizes particularly well theadvantages of noble metal catalysts, in particular those based onplatinum, pallladium and/or iridium, namely high activity and goodselectivity.

For the purposes of the present invention, preference is given to usingaromatic nitro compounds having one or more nitro groups and from 6 to18 carbon atoms, for example nitrobenzenes, such as o-, m- orp-nitrobenzene, 1,3-dinitrobenzene, nitrotoluenes, e.g. 2,4- or2,6-dinitrotoluene, 2,4,6-trinitrotoluene, nitroxylenes, e.g.1,2-dimethyl-3-, 1,2-dimethyl-4-, 1,4-dimethyl-2-, 1,3-dimethyl-2-,2,4-dimethyl-1- and 1,3-dimethyl-5-nitrobenzene, nitronaphthalenes, e.g.1- or 2-nitronaphthalene, 1,5 and 1,8-dinitronaphthalene,chloronitrobenzenes, e.g. 2-chloro-1,3- or 1-chloro-2,4-dinitrobenzene,o-, m- or p-chloronitrobenzene, 1,2-dichloro-4-, 1,4-dichloro-2-,2,4-dichloro-1- and 1,2-dichloro-3-nitrobenzene, chloronitrotoluenes,e.g. 4-chloro-2-, 4-chloro-3-, 2-chloro-4- and 2-chloro-6-nitrotoluene,nitroanilines, e.g. o-, m- or p-nitroaniline; nitroalcohols, e.g.tris(hydroxymethyl)nitromethane, 2-nitro-2-methyl- or2-nitro-2-ethyl-1,3-propanediol, 2-nitro-1-butanol or2-nitro-2-methyl-1-propanol, and also any mixtures of two or more of thenitro compounds mentioned.

Preference is given to hydrogenating aromatic nitro compounds,preferably mononitrobenzene, methylnitrobenzene or methylnitrotoluene,and in particular 2,4-dinitrotoluene or its industrial mixtures with2,6-dinitrotoluene, with these mixtures preferably having up to 35percent by weight, based on the overall mixture, of 2,6-dinitrotoluenewith fractions of from 1 to 4 percent of vicinal DNT and from 0.5 to1.5% of 2,5- and 3,5-dinitrotoluene, to give the corresponding amines.

The invention is illustrated by the following examples.

EXAMPLE 1 AND COMPARATIVE EXAMPLE 2

Hydrogenation of Dinitrotoluene

In a 5 l jet loop reactor equipped with a circulation pump, jet nozzle,internal tube and heat exchanger, the dinitrotoluene was hydrogenatedover a supported nickel catalyst having an average particle size of from5 to 10 μm at 120° C. and 25 bar. The activity of the catalyst wasconstantly monitored by gas chromatography analysis samples and whennecessary, further catalyst was metered in. The concentration ofcatalyst was 3% by weight, based on the reaction mixture.

The removal of the catalyst from the reaction mixture was effectedeither by a membrane filter (inventive) or by a settler (comparative).

EXAMPLE 1 Membrane Filtration

A cylinder made of highly porous ceramic having a length of 750 mm and alengthwise channel having a diameter of 6 mm, on whose surface theactual filtering membrane made of zirconium dioxide having a pore sizeof 50 nm was applied. The suspension to be treated flowed with a fluxrate of 4 m/s in the channel along the membrane, and a partial streampassed through the membrane as permeate and was removed through theceramic support material. The transmembrane pressure was 2 bar, thepermeate flux 440 l/m²/h.

COMPARATIVE EXAMPLE 2 Gravitational Separator (Settler)

A partial stream of the circulation stream (reactor-circulationpump-nozzle) was diverted using the initial pressure of the circulationpump to the lower part of a settler and passed from there back into thereactor according to the mass flows without an additional conveyingelement.

The actual reactor effluent (settler effluent) flowed upwardly through aseparating tube at an angle of 55° and, controlled by the liquid leveland gas content in the reactor, was discharged through a pressure reliefvalve. The flux ratios in the separator lamella were set so that allparticles larger than 1 μm were separated and fell down to the bottom ofthe settler, from where they were transported by the circulation flow(settler recycling) back into the reactor. Smaller particles were passedout with the end product.

Results

In two series of experiments, the reactor was operated once equippedwith a gravity separator (comparative experiment) and once equipped withmembrane filtration (inventive).

Using a gravity separator, a space-time yield of 250-350 kg ofTDA/(m³·h) was achieved over a period of 4 weeks. 400 to 600 g ofcatalyst were consumed per metric ton of TDA.

Using membrane filtration, a space-time yield of 400 to 500 kg ofTDA/(m³·h) was achieved over a period of 3 months. 350 to 450 g ofcatalyst were consumed per metric ton of TDA.

EXAMPLES 3 AND 4

The devices described in Example 1 for hydrogenation and catalystremoval were used and the effectiveness of different catalysts atdifferent reaction temperatures in the hydrogenation of nitrobenzene wastested.

EXAMPLE 3

The catalyst from Example 1 was used in a concentration of 2% by weight,based on the reaction mixture. The transmembrane pressure was 1 bar, thepermeate flux 200 l/m²/h.

At 140° C., a space-time yield of 800 kg/(M³·h) was achieved at aselectivity of 99.7%, and at 180° C., a space-time yield of 1800kg/(m³·h) was achieved at a selectivity of 98.3%.

EXAMPLE 4

A catalyst comprising 5% by weight of platinum and 2% by weight of ironon activated carbon having an average particle size of from 20 to 30 μmin a concentration of 2% by weight, based on the reaction mixture, wasused. The transmembrane pressure was 1 bar, the permeate flux 200l/M²/h.

At 140° C., a space-time yield of 1500 kg/(m³·h) at a selectivity of99.82% was achieved. At 180° C., a space-time yield of 2200 kg/(m³·h) ata selectivity of 99.6% was achieved.

1. A process for preparing amines by catalytic hydrogenation ofnitroaromatics and subsequent removal of the catalysts from the reactionmixture, which contains at least one aromatic amine and water, whichcomprises carrying out the removal of the catalysts continuously bymeans of membrane filtration, which is carried out at a pressure on thesuspension side of from 5 to 50 bar, a pressure difference between thesuspension side and the permeate side of at least 0.3 bar and a fluxrate on the suspension side of from 1 to 6 m/s.
 2. A process as claimedin claim 1, wherein the pressure on the suspension side is from 10 to 30bar.
 3. A process as claimed in claim 1, wherein the filter membrane hasa pore diameter in the range from 10 nm to 20 μm.
 4. A process asclaimed in claim 1, wherein the hydrogenation is carried out in a jetloop reactor.
 5. A process as claimed in claim 1, wherein thehydrogenation is carried out in a jet loop reactor having an externaland an internal circuit.
 6. A process as claimed in claim 1, wherein thecatalysts used comprise metals of transition group VIII of the PeriodicTable on supports.
 7. A process as claimed in claim 1, wherein thecatalysts used comprise platinum, palladium and/or iridium catalysts oncarbon supports.
 8. A process as claimed in claim 1, wherein thehydrogenation is carried out in a jet loop reactor having an externaland internal circuit, the catalysts used comprise platinum, palladiumand/or iridium catalysts on carbon supports and the membrane filter islocated in the external circuit of the reactor.